Vortex pinning and flux creep in Bi₂Sr₂(Ca_1-xR_x)Cu₂O_8+y(R=Y,Gd)

Abstract

We present magnetisation measurements on powders of BizSrz(Cal. R , ) C U ~O ~+ ~ (R = Y,Gd). The observed magnetic relaxations and the field dependence o? the pinning force have been described in terms of a thermally activated flux-creep model without the need to consider phase transitions of the vortex system. We have found that the intra-grain J, decreases rapidly with rare earth or Y substitution. Even in the concentration range for which T, remains constant, J, decreases: J,(x=O)/J,(x=.32) = 5.7.